Rise in CO2 affects soil water transport through repellency

Several studies have shown improved soil stability under elevated atmospheric CO2 caused by increased plant and microbial biomass. These studies have not quantified the mechanisms responsible for soil stabilisation or the effect on water relations. The objective of this study was to assess changes in water repellency under elevated CO2. We hypothesised that increased plant biomass will drive an increase in water repellency, either directly or through secondary microbial processes. Barley plants were grown at ambient (360 ppm) and elevated (720 ppm) CO2 concentrations in controlled chambers. Each plant was grown in a separate tube of 1.2 m length constructed from 22 mm depth x 47 mm width plastic conduit trunk and packed with sieved arable soil to 55% porosity. After 10 weeks growth the soil was dried at 40A degrees C before measuring water sorptivity, ethanol sorptivity and repellency at many depths with a 0.14 mm radius microinfiltrometer. This provided a microscale measure of the capacity of soil to rewet after severe drying. At testing roots extended throughout the depth of the soil in the tube. The depth of the measurement had no effect on sorptivity or repellency. A rise in CO2 resulted in a decrease in water sorptivity from 1.13 +/- 0.06 (s.e) mm s(-1/2) to 1.00 +/- 0.05 mm s(-1/2) (P <0.05) and an increase in water repellency from 1.80 +/- 0.09 to 2.07 +/- 0.08 (P <0.05). Ethanol sorptivity was not affected by CO2 concentration, suggesting a similar pore structure. Repellency was therefore the primary cause of decreased water sorptivity. The implications will be both positive and negative, with repellency potentially increasing soil stability but also causing patchier wetting of the root-zone.